Rapid urbanization causes stress in limited water resources. Water managers need quantified streamflow as high, medium and low flow regions to assess and manage water resources. This requires calibrated and validated models together with a rationalized method for the selection of a proper model, components, data, temporal resolution, objective function, and performance criteria. The current preference is to use process-based models for streamflow modelling. The widely used process-based HEC-HMS 4.2.1 version model was chosen to simulate daily streamflow at Baddegama watershed (749 km 2 ) while using 2007 to 2012 period for calibration, and 2013 to 2017 for verification. Simple canopy, initial deficit and constant loss, SCS direct runoff, and recession baseflow were selected as model components. Semi-automatic optimization was done with RMSE objective function. Model was calibrated and validated with RMSE of 3.0 mm/day and 3.5 mm/day. Model with RMSE value of 6.2 mm/day for high flows and 2.3 mm/day for medium flows indicate better capability of model on flooding management and water resource. The HEC model showed very good matching of daily flows in the high and intermediate regions but reflected a poor matching in the low flow region. Hence the use of HEC-HMS model for the considered regions can be recommended for flood and water resources management. The use of HECHMS for low flow management must be with caution.